1. Field of the Invention
The present invention refers to a particle detector, especially to a particle detector for detecting positive or negative ions.
2. Description of the Related Art
Such detectors have been used in many fields of technology. In particular, ion detectors are used in mass spectrometry.
Conventionally, electrically charged particles may be detected by a traditional ion detector, the most well-known one of which is a scintillation detector usually called “Daly detector”. A conventional Daly detector is described in N. R. Daly, “Scintillation type mass spectrometer ion detector”, Review of Scientific Instruments, Vol. 31 No. 3, 264-267, March 1960. This conventional detector assembly consists of a −40 kV high voltage conversion dynode and an aluminum-plated scintillating layer equipped with a photomultiplier tube (PMT). The surfaces of the conversion dynode and the scintillating layer face each other. While entering the detection region in a direction parallel to the surface of the dynode, positive ions are deflected and accelerated towards the surface of the conversion dynode by the strong electric field and eject secondary electrons while impinging on the dynode surface. The ejected electrons, because of the strong repulsion Coulombic force of the conversion dynode, will accelerate towards the low potential scintillation layer and induce luminescence for PMT detection. However, the Daly detector is generally only used for detecting positively charged particles but not for negatively charged particles due to the negative electric polarity of the conversion dynode.
In order to apply scintillation-type detectors to anion detection, it has been proposed in A. Pramann et al., “A new scintillation-type time-of-flight anion detector”, Review of Scientific Instruments, Vol. 72 No. 8, 3475-3476, August 2001, to use an arrangement of two metal grids to generate the secondary electrons. Both grids are arranged perpendicularly to the direction of the anion beam. The first grid is electrically grounded, whereas the second grid, which serves as a conversion dynode, is biased at +2 kV. When entering the area between the two grids, the anions are accelerated onto the second grid, where the secondary electrons are generated. Behind the second grid, a highly positively biased scintillating surface is arranged which attracts and detects the secondary electrons. Although this detector can detect anions, the application of this type of scintillating detector is mainly limited by its sensitivity and mass working-range (<1,000 m/z in all literature reports). Moreover, no literature reports the detection of cations by using this detector.
A combined cation and electron detector has been disclosed in U.S. Patent Application Publication 2004/0262531 A1. This detector consists of several charged plates which are arranged in parallel to each other and in parallel to the initial beam path as well as a scintillator located behind the plates and biased at +10 kV. Alternatively, the charged plates can be replaced by a charged cylinder. In the cation detection mode, the charged plates or the cylinder are biased at −2 kV, which attracts the cations and results in the generation of secondary electrons when the cations impinge on the surface. The secondary electrons are then accelerated towards the scintillator. In the electron detection mode, the charged plates or the cylinder are biased at +500 V. Although some electrons might strike the plates or cylinder, the majority of them pass through and impinge on the scintillator directly. However, a detection of heavier anions is not disclosed.
Thus, a particle detector which can detect both positive and negative ions without any change in detector condition has not been proposed and is highly desirable.